# Collection loops, indexes, reverse, and where

for can iterate collection values, expose an index, start that index elsewhere, reverse the traversal, or filter with where.

Last updated: 2026-09-09

Package: language | Status: stable

Tags: at, complete-program, cpu, for, in, language, loops, reverse, to, where, yeho

Use collection iteration when you care about elements rather than storage positions.

```yh
for item in collection { ... }
for item at i in collection { ... }
for item at i = 1 in reverse collection where condition { ... }
for key to value in map { ... }
```


## List runtime

Use collection iteration when you care about elements rather than storage positions.

Verification: type checked

```yh
list of int values = [2, 4]
values.Add(6)
values[1] = values[1] + 1

int total = 0
for value in values
{
    total = total + value
}

Console.Log(Text.Format("{0}:{1}:{2}", Text.From(values.count), Text.From(values[1]), Text.From(total)))

values.Clear()
values.Add(9)
values.Add(11)
Console.Log(Text.Format("clear:{0}:{1}:{2}", Text.From(values.count), Text.From(values[0]), Text.From(values[1])))



```

Save this beside start.yh as project.mech:

```toml
package = "tests.compiler.kyber.listRuntime"
version = "0.1.0"
backend = "kyber"
osTarget = "windows"
archTarget = "x64"
selfContainedApp = false




```

Extract the example archive beside the yeho and dolphin checkouts. This example requires those source dependencies and a current developer compiler.

```sh
./yeho/build/dolphin-metal/yehoc ./api-examples/language--collection-for/complete --backend cpu-oracle -o /tmp/yeho-example
/tmp/yeho-example
```


reverse appears after in and before the collection expression.

Map loops can bind key to value.

Source: yeho/parser_loops.cpp; language-core.md §8.3

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